Light Activated Relay

This is same circuit as above with the addition of a photo resistor to
trigger the flip flop instead of a push button. The bias resistor in series
with photo resistor was chosen so that sufficient voltage is present at
the base of the 2N3904 to supply current to the circuit in ambient lighting conditions. The circuit should toggle when the photo
resistor is hit by a flashlight beam or other fast changing light source.
Slow changes in light intensity will have no effect unless the light gets
too bright to maintain sufficient bias for the 2N3904.

Flashing Neons (NE-2 / NE-51)

In this circuit, one, two or three neon indicator bulbs can be made to
flash in sequence at rates determined by the R and C values. In the single
stage circuit, using one lamp, the capacitor charges through the resistor
until the ionization potential of the neon is reached (about 70 volts) and
then discharges quickly through the lamp until the voltage falls below what
is needed to sustain the lamp which is approximately 45 volts. The cycle
then repeats at a rate of about 3 Hz for values shown. Smaller R or C values
increase frequency, larger values decrease frequency. All capacitors should
be the non-polarized variety with a 100 volt or more rating. For more than
3 stages, the lamps may need to be matched for similar turn-on voltages.

12 Stage Neon Sequencer (NE-2 / NE-51)

This circuit is similar to the LED clock using 12 neon indicator lamps
instead of LEDs. It operates from 2 high capacity ni-cad cells (2.5 volts)
which keep it going for a couple weeks. High voltage (70 volts) for the neon
lamps is obtained from a small switching power supply using a 74HC14 Schmitt
trigger squarewave oscillator, high voltage switching transistor, and 10 mH
high Q inductor. Most any small PNP transistors can be used that have a C/E
voltage rating of 80 or more. The inverter stage (pins 5,6) is not needed
and is just an extra stage. An adjustable low frequency oscillator made from
two of the inverter stages generates the clock signal for the 74HCT393 binary
counter. In this circuit, the timing capacitor should be non-polarized since
the capacitor will charge in both directions, so two 6.8 uF tantalum caps were
used back to back which yields about 3.3 uF. The 75K resistor in series with
pin 1 limits the current through the input protection diodes when the
capacitor voltage exceeds the supply voltage. This resistor may not be
necessary with small capacitors at low voltage but was added as a precaution.
The binary counts are decoded into 1 of 12 outputs by the 74HCT138 decoders
and operates the same way as in the 28 LED clock circuit. The sequence can
be extended to 16 by omitting the reset circuit and tying pins 2 and 13 of
the counter to ground.

Constant Current Battery Charger

A simple method of charging a battery from a higher voltage battery
is shown in the circuit below to the left. Only one resistor is needed
to set the desired charging current and is calculated by dividing
the difference in battery voltages by the charge current. So, for example
if 4 high capacity (4000 mA hour) ni-cads are to be charged at 300 mA from a
12 volt battery, the resistor needed would be 12-(4*1.25)/0.3 = 23.3
ohms, or 22 ohms which is the nearest standard value. The power rating
for the resistor is figured from the square of the current times the
resistance or (0.3)^2 * 22 = 2 watts which is a standard value but
close to the limit, so a 5 watt or greater value is recommended.

The circuit below (right) illustrates a constant current source used to
charge a group of 1 to 10 ni-cad batteries. A 5K pot and 3.3K resistor are
used to set the voltage at the emitter of the TIP 32 which establishes the
current through the output and 10 ohm resistor. The emitter voltage will be
about 1.5 volts above the voltage at the wiper of the pot, or about
1/2 the supply voltage when the wiper is in the downward most position.
In the fully upward position the transistors will be turned off and
the current will be close to zero. This yields a current range of
0 to (0.5*input)/10 or 0 to 850 milliamps using a 17 volt input.
This produces about 7 watts of heat dissipation at maximum current
for the 10 ohm resistor, so a 10 watt or greater rating is needed.
The TIP 32 transistor will also dissipate about 7 watts if the output
is shorted and needs to be mounted on a heat sink. If more than 4
cells are connected, the maximum current available will decrease and
limits the current setting to about 100 milliamps for 10 cells.
The usual charge rate for high capacity (4AH) 'D' cells is 300 to 400
milliamps for 14 hours and 100 milliamps for (1.2AH) 'C' or 'D' cells.
For small 9 volt batteries the charge rate is 7 milliamps for 14 hours
which would be difficult to set and probably unstable, so you could
reduce the range to 0-20 mA by using a 750 ohm resistor in place of the
10. The charge current can be set by connecting a milliamp meter across the
output (with the batteries disconnected) and then adjusting the control to
the desired current, or by monitoring the voltage across the 10 ohm resistor
(1 volt = 100 mA) or (1 volt = 1.33 mA using a 750 ohm resistor). The
current control should be set to minimum (wiper in uppermost position)
before power is applied, and then adjusted to the desired current.

The circuit (lower right) illustrates using a LM317 variable voltage
regulator as a constant current source. The voltage between the adjustment
terminal and the output terminal is always 1.25 volts, so by connecting
the adjustment terminal to the load and placing a resistor (R) between the
load and the output terminal, a constant current of 1.25/R is established.
Thus we need a 12 ohm resistor (R) to get 100mA of charge current and a
1.2 ohm, 2 watt resistor for 1 amp of current. A diode is used in series
with the input to prevent the batteries from applying a reverse voltage
to the regulator if the power is turned off while the batteries are still
connected. It's probably a good idea to remove the batteries before
turning off the power.

120VAC Lamp Chaser

This circuit is basically the same as the 10 channel LED sequencer with
the addition of solid state relays to control the AC lamps. The relay
shown in the diagram is a Radio Shack 3 amp unit (part no. 275-310) that
requires 1.2 volts DC to activate. No current spec was given but I assume
it needs just a few milliamps to light the internal LED. A 360 ohm resistor
is shown which would limit the current to 17 mA using a 9 volt supply.
I tested the circuit using a solid state relay (of unknown type) which
required only 1.5 mA at 3 volts but operates up to 30 volts DC and a much
higher current. The chaser circuit can be expanded up to 10 channels with
additional relays and driver transistors. The 4017 decade counter reset line
(pin 15) is connected to the fifth count (pin 10) so that the lamps sequence
from 1 to 4 and then repeat. For additional stages the reset pin would be
connected to a higher count.

Game Show Indicator Lights (Who's First)

The circuit below turns on a light corresponding to the first of several
buttons pressed in a "Who's First" game. Three stages are shown but the
circuit can be extended to include any number of buttons and lamps.

Three SCRs (silicon controlled rectifiers) are connected with a common
cathode resistor (50 ohm) so that when any SCR conducts, the voltage
on the cathodes will rise about 7 volts above the voltage at the junction
of the 51K and 1K ohm resistors and prevent triggering of a second
SCR. When all lamps are off, and a button is pressed, the corresponding
SCR is triggered due to the voltage at the divider junction being
higher than the cathode. Once triggered, the SCR will remain conducting
until current is interrupted by the reset switch. Or, you can just turn
the power off and back on.

A 50 ohm, 5 watt resistor was selected to produce a 10 volt drop at 200 mA
when a single 25 watt lamp comes on. Higher wattage lamps would require
a lower value resistor, and visa versa. For example to use 60 watt lamps
and maintain the 10 volt drop, the peak current would be 60/160 = 375 mA
and the resistance would be E/I = 10/.375 or about 27 ohms at 3.75 watts.
The SCRs are "Sensitive Gate' types which trigger on about 200 uA and
the gate current is around 1.5 mA when the first button is pressed. The
1N914 diodes in series with the buttons gates are used to prevent a
reverse voltage on the gate when a button is pressed after an SCR is
conducting. The two 51 ohm resistors will be fairly large in physical
size (compared to a 1/4 watt size) and should be rated for 5 watts of power
or more. Use caution and do not touch any components while the circuit is
connected to the AC line.

Adding a Buzzer:

The relay shown in parallel with the 50 ohm cathode resistor can be used to
momentarily power a buzzer with an external circuit through the contacts.
The 1000 uF capacitor causes the relay to energize for about one second,
longer times can be obtained with a larger capacitor.

Pinewood Derby Finish Line Lamps

The finish line circuit below detects the first of three cars to cross the
line and illuminates a 25 watt 120 VAC lamp indicating the winning lane.
Three photo transistors are used which can be embedded into the track with
a light shining down onto the finish line so that as the car crosses over
the sensor, the light is blocked, activating the relay and lighting the
lamp for the appropriate track. The light source should be an incandescent
type, florescent lights may not work due to low infra-red content. The
circuit was tested using a 100 watt incandescent light fixture about 3 feet
above the photo transistors.

The photo transistors are connected so that a logic low (0 volts) normally
appears at the input to a NAND gate and as a car crosses the line blocking
light to the transistor the logic level will move high (+6 volts).
The resulting logic low level from the output of the gate (3 input NAND) is
fed to a SET/RESET latch made from two dual input NAND gates (1/2 of a 74HC00)
the (logic high) output of which controls the MPS2222A buffer transistor and
solid state relay. The inverted output of the latch (logic low) is connected
back to the remaining two (3 input NAND gate) inputs locking them out. Two
extra 74HC00 gates are not used and should have their inputs (pins 9,10,12,13)
connected to ground to avoid possible oscillation. The circuit is reset with
a momentary push button connected to the reset side of each latch. The reset
button may need to be pressed after power is first applied. Components for
the circuit may be obtained from Radio Shack, however the RSU numbers may
need to be special ordered or obtained from another source. The 74HC00 and
74HC10 are CMOS parts and should be handled carefully to avoid possible damage
from static electricity. You may want to use IC sockets so the wiring can be
completed before the ICs are inserted into the sockets. You can briefly touch
a grounded surface (computer chassis or other metal ground surface) just
before handling CMOS circuits to reduce the possibility of damage from static
electricity.

Notes:

All ground symbols are connected to the negative side of the battery.
All +6 points are connected to the positive side of the battery.
Transistors and relays for #2 and #3 lights are not shown but are
connected the same as shown for #1.
A small LED may be substituted for the solid state relay (pins 3,4)
for testing the circuit before the relays are installed.
Pins 8,6 and 12 of the 74HC10 should read +6 volts after reset is
pressed and light is shining on the photo transistors.
Pins 1,9 of the top 74HC00 and pin 1 of the lower 74HC00 should read
+6 volts with the reset button released. The same pins should
read 0 volts with the button pressed.
Pins 2,6,10,11 of upper 74HC00 and 2,6 of lower should read 0 volts
after reset button is released and photo transistors are illuminated.
Pins 3,4,8,12 of upper 74HC00 and pins 3,4 of lower should read +6
after reset button is released.
Pins 9,10,12,13 of lower 74HC00 should be grounded